This invention relates to electronic data communications systems and, in particular, to a non-contact system for two-way communication between a station and a portable data card. Non-contact communications systems do not require the portable data card to be inserted into the station, but allow data transfer to be effected when the card is brought into close proximity with the station. Such data communication systems have been proposed for use in, for example, security systems, bank transaction systems and so on.
In U.S. Pat. No. 3,299,424 (Vinding) there is disclosed an interrogator-responder identificaton system in which the responder is identified when inductively coupled to the interrogator. The inductive coupling is achieved by means of resonant circuits tuned to the same frequency within the responder and interrogator, thereby enabling non-contact communication between the two.
In a preferred embodiment the responder is self-powered, deriving its dc supply voltage by rectifying a portion of the induced interrogator signal.
Data stored within the responder is read or identified, by the interrogator by means of a detuning or loading circuit coupled to the responder through a switch means. The switch means is activated in response to the stored data so as to load the responder resonant circuit, thereby decreasing its interaction with the interrogator resonant circuit. Consequently, the varying loading effect of the responder on the interrogator resonator circuit may be interpreted in terms of the responder data. For example, a signal corresponding to the responder data may be transmitted to the interrogator by amplitude- or phase-modulating the resonant frequency signal of the interrogator.
Whilst Vinding discloses a system in which a responder, self-powered by means of a signal transmitted by an interrogator, transmits data to the interrogator, there is no provision for writing data from the interrogator to the responder.
In U.S. Pat. No. 4,517,563 (Diamant), an identification system is disclosed similar to that of Vinding (above) in which an active transponder (a stationary reader) reads data stored within the memory of a passive transponder (a portable identifier). Communication between the reader and the identifier is achieved by means of tuned resonant circuits in both the reader and the identifier, thereby enabling communication to take place without physical contact between the two. The portable identifier is not equipped with its own independent power supply, but operates on power generated as a result of the inductive coupling between the resonant circuits in the transponders. Thus, Diamant also discloses a system in which power is transmitted from the reader to the identifier and data is transmitted from the identifier to the reader. However, there is no provision for transmitting data from the reader to the identifier.
In U.S. Pat. No. 4,605,844 (Haggan) there is disclosed a computerized data transfer system in which both power and data can be inductively transferred from a stationary reader to a portable card and, moreover, data can also be transferred from the card to the reader. Power and data in both directions are transferred between the reader and the card by means of three separate transformer coils located in both the reader and the card which are inductively coupled when the card is brought into very close proximity with the reader. Thus, although this arrangement permits two-way data communication between a reader and a portable card, it requires separate transformer coupling both for power transfer and also for data transfer in each direction. Moreover, owing to the poor inductive coupling inherent in such a system, such an arrangement will work only if the portable card is brought within extremely close proximity to the stationary reader.